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    Credit: Jameson Simpson

Walk into the atrium lobby of a new commercial building and you may marvel at the extensive curtainwall glazing or large-scale art installation. What shouldn’t catch your eye is the maze of mechanical infrastructure that typically looms overhead in building spaces. In a multistory space, many designers will strive to minimize visual interruptions to preserve its openness, but they also know that conditioning such an area can require a lot of equipment and a lot of energy.

Radiant flooring may be the solution to both problems. Dan Nall, FAIA, a senior vice president in the New York office of WSP Flack + Kurtz, says that radiant floors offer many advantages over traditional systems such as forced air: “There is a significant opportunity both for energy efficiency as well as the avoidance of the architectural impact of [mechanical] equipment and distribution that might normally be required in large, highly glazed spaces.”

Radiant floor heating relies on both the principles of infrared radiation and convection. A large thermal mass, such as a concrete slab, is heated and then radiates its warmth to nearby people and objects; areas closest to the floor feel the effects the most. Ventilation, which is required to supply fresh air to occupants and to dehumidify the space in the case of radiant cooling, helps distribute the heat.

A hydronic system is the most common and cost-effective radiant heating floor system in climates that have a long heating season, according to the U.S. Department of Energy. Water, the heat-transfer medium, circulates through plastic tubing—typically cross-linked polyethylene (PEX)—that is fastened to wire-welded mesh or rigid insulation board atop the structural slab and embedded in a high-density concrete topping slab.

The tubing runs back and forth in 8- to 12-inch spaced loops in lengths of up to 300 feet, which, Nall estimates, covers about 200 to 300 square feet. Beyond 300 linear feet, the heating medium begins to lose its effectiveness, says Merlin Maley, AIA, a project manager at Denver-based RNL Design. Designers, as a result, must “draw zones similar to a ductwork plan with tubing and manifold locations,” he says. Each manifold can serve six to 12 loops of tubing.

At Transpo’s Emil “Lucky” Reznik Administration, Maintenance, and Operations Facility in South Bend, Ind., RNL and local consultant firm M/E Design Services designed eight radiant heat zones in the 26,000-square-foot, 76-bus-capacity maintenance garage. Four snowmelt zones are also connected to the system, which runs on a gas boiler and an electric boiler.

Radiant systems used outside the building employ water with an antifreeze additive such as glycol, Nall says. The water inside the tubing is warmed between 78 F and 80 F, which isn’t “super hot,” he adds, “but when you have an enormous area at that temperature, it can provide the amount of heating that can significantly improve comfort.” At Transpo’s facility, radiant heating meant that the massive concrete slab supporting the vehicles could be dual-purposed, “which cuts down on big air handlers,” Maley says. For the mechanics, who spend much of their time close to the ground underneath vehicles, “not being on a cold concrete slab makes a big difference.”

Radiant floor systems make sense in programmatic spaces that will not experience significant change in use or layout, says John Schuyler, AIA, a principal at New York–based FXFowle. Once installed, the tubing “wants to be fairly permanent,” he says. For SAP Americas’ headquarters expansion in Newtown Square, Pa., FXFowle and WSP Flack + Kurtz collaborated on the radiant floor system that covers the 210,000-square-foot building’s four-story, 460-foot-long circulation spine, which Nall says is an ideal space for the technology. “The most obvious slam dunk for these [systems] are atriums … and [other] tall spaces with a lot of glass,” he says. Here, conditions can swing quickly depending on solar loading. “The atrium spaces can feel cold even if we’re trying to move [heated] air through the space,” Schuyler says. “Radiant heating helped improve user comfort close to the ground.”

A radiant floor was also installed in the four-story, 5,000-square-foot galleria space at the Rochester Institute of Technology’s new 84,000-square-foot Golisano Institute for Sustainability (GIS) building, designed by FXFowle and local firm SWBR Architects. The density of the tubing means that coordinating the layout for furniture, structural supports, and other infrastructure before installing the flooring is critical, says Mark Maddalina, AIA, SWBR’s manager of sustainable design. “We stand no chance of making [floor] penetrations later on,” he says.

To cap off a successful radiant floor, a finish with a high thermal conductivity should be used. The topping slab, left exposed, obviously works, such as in the case at Transpo’s facility, as does ceramic tile and terrazzo, the latter of which the SAP and GIS buildings use. Carpet, which GIS uses in two upper-floor collaboration spaces that incorporate radiant flooring, can work, but designers must specify the carpet type and cushion carefully, says SWBR interior designer Michele Tuck. She recommends using a system with an R-value under 4, which means a low-pile carpet and a cushion no thicker than 3/8 inch. However, the effectiveness of the radiant floor decreases as the insulation value of the finish material increases.

Maintaining a radiant floor requires the occasional replacement of the valves and pumps that circulate the water through the tubing. If installed and tested properly, the embedded tubing should have little risk of deterioration or leakage, Nall says. “It should be good for the life of the building.” A 2- to 3-inch concrete clearance between the tubing and finished floor surface will protect the tubing from ultraviolet light and sharp implements, he says, while precluding tubing splices in the slab will minimize the risk of leakage.

Though the initial cost of radiant floor heating is more expensive than other conditioning systems, Schuyler says, “The long-term operating costs are significantly lower, as is the energy consumption. For any client who is able to have a longer view and financial horizon, it’s going to make sense.” While most buildings supplement radiant flooring with other heating methods, such as forced air, a well-designed building management system will help ensure that the ventilation, temperature-control, and dehumidification systems work together efficiently.

At SAP, Nall estimates that the circulation spine requires only 15 to 25 percent of the air volume that would otherwise be needed if the space relied entirely on forced air. And since heating and circulating air requires five to eight times more energy than heating and circulating water, Nall says, radiant flooring can produce significant savings. Using an alternative energy source for the hydronic system can provide further savings. The SAP headquarters takes advantage of a geothermal-coupled system, while GIS uses both geothermal energy and rejected heat from fuel cells to power its radiant flooring system.

Though its radiant flooring system uses conventional energy sources—gas and electricity—Transpo still pays significantly less money to heat its new facility than it did at its previous facility. By running the electric boiler at night during off-peak hours, Transpo was able to negotiate a low, long-term price on the electricity, Maley says. While Transpo’s old facility cost $1.75 per square foot to heat through forced air, the new facility, which is double the size of the former, costs $0.75 per square foot to heat, a 57 percent savings.

And for some commercial projects, economic savings is only an ancillary benefit of radiant flooring. Pragmatically speaking, conditioning the immediate space that building users occupy makes sense, says Brian Danker, an associate at Rochester, N.Y.–based M/E Engineering, the mechanical engineer for the GIS project. “We never looked at the radiant floor as an energy-saving scenario,” he says, “but really as a comfort benefit to the project.”